Volatile memory elements with soft error upset immunity

ABSTRACT

Memory elements are provided that exhibit immunity to soft error upset events when subjected to high-energy atomic particle strikes. The memory elements may each have ten transistors including two address transistors and four transistor pairs that are interconnected to form a bistable element. Clear lines such as true and complement clear lines may be routed to positive power supply terminals and ground power supply terminals associated with certain transistor pairs. During clear operations, some or all of the transistor pairs can be selectively depowered using the clear lines. This facilitates clear operations in which logic zero values are driven through the address transistors and reduces cross-bar current surges.

This application is a continuation of patent application Ser. No.13/411,436, filed Mar. 2, 2012, which is a continuation of Ser. No.12/686,597, filed Jan. 13, 2010, which is a continuation of patentapplication Ser. No. 12/407,762, filed Mar. 19, 2009, which are herebyincorporated by reference herein in their entireties. This applicationclaims the benefit of and claims priority to patent application Ser. No.13/411,436, filed Mar. 2, 2012, patent application Ser. No. 12/686,597,filed Jan. 13, 2010, now U.S. Pat. No. 8,154,912 and patent applicationSer. No. 12/407,762, filed Mar. 19, 2009, now U.S. Pat. No. 7,872,903.

BACKGROUND

This invention relates to volatile memory elements, and moreparticularly, to volatile memory elements that demonstrate soft errorupset immunity.

Integrated circuits often contain volatile memory elements. Typicalvolatile memory elements are based on cross-coupled inverters (latches).A volatile memory element retains data only so long as the integratedcircuit is powered. In the event of power loss, the data in the volatilememory element is lost. Although nonvolatile memory elements such asmemory elements based on electrically-erasable programmable read-onlymemory technology are not subject to data loss in this way, it is oftennot desirable or possible to fabricate nonvolatile memory elements aspart of a given integrated circuit.

As a result, volatile memory elements are often used. For example,static random-access memory (SRAM) chips contain SRAM cells, which are atype of volatile memory element. Volatile memory elements are also usedin programmable logic device integrated circuits.

Volatile memory elements are subject to a phenomenon known as soft errorupset. Soft error upset events are caused by cosmic rays and radioactiveimpurities embedded in integrated circuits and their packages. Cosmicrays and radioactive impurities generate high-energy atomic particlessuch as neutrons and alpha particles. The memory elements containtransistors and other components that are formed from a patternedsilicon substrate. When an atomic particle strikes the silicon in thememory element, electron-hole pairs are generated. The electron-holepairs create a conduction path that can cause a charged node in thememory element to discharge and the state of the memory element to flip.If, for example, a “1” was stored in the memory element, a soft errorupset event could cause the “1” to change to a “0.”

Upset events in an integrated circuit corrupt the date stored in thememory elements and can have serious repercussions for systemperformance. In certain system applications such as remote installationsof telecommunications equipment, it is extremely burdensome to repairfaulty equipment. Unless programmable logic devices and other integratedcircuits demonstrate good immunity to soft error upset events, they willbe unsuitable for these types of applications. At the same time, careshould be taken to ensure that too much circuit area is not consumed bythe volatile memory elements on an integrated circuit and that thevolatile memory elements exhibit good performance characteristics.

It would therefore be desirable to be able to improve the soft errorupset performance of volatile memory elements in integrated circuitssuch as programmable logic device integrated circuits.

SUMMARY

Integrated circuits with memory elements are provided. The integratedcircuits may be programmable integrated circuits, memory chips, or anyother suitable integrated circuits. The memory elements may be used asstatic random-access memory (SRAM) or, in programmable integratedcircuits, the memory cells may be used as configuration random-accessmemory (CRAM).

Each memory element may have a pair of address transistors and fourtransistor pairs. Each transistor pair may have an n-channel and ap-channel transistor connected in series at a respective output node.The output signals from the output nodes may be provided to twodifferent transistor gates, so that the transistor pairs serve asinverter-like circuits with distributed inputs. This type of arrangementallows the memory elements to exhibit good immunity to soft error upsetevents.

Data write operations may be performed using complement data lines.Logic zero values may be placed on the complement data lines to write alogic one into a memory element. During these logic one writeoperations, logic zeros are driven through the address transistors ontothe nodes of the second and fourth transistor pairs. The addresstransistors may be implemented using n-channel transistors that readilypass logic zero values during data write operations. Clear operationsmay be performed by driving logic ones onto the output nodes of thesecond and fourth address transistors. During these operations, theaddress transistors may have difficulty in passing the logic one values.To ensure that clear operations are successful, the clear lines may beused to temporarily depower (weaken) some or all of the transistor pairsduring clearing. The depowered transistor pairs exhibit minimalcross-bar current, which reduces surges in clear currents in the memoryarray. Once the memory element has been cleared, the clear lines may beused to repower the memory element.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative integrated circuit that maycontain random-access memory cells in accordance with an embodiment ofthe present invention.

FIG. 2 is a diagram of an illustrative array of memory cells inaccordance with an embodiment of the present invention.

FIG. 3 is a diagram of an illustrative memory cell in accordance with anembodiment of the present invention.

FIG. 4 is a flow chart of illustrative steps involved in clearing anarray of memory cells of the type shown in FIG. 3 in accordance with anembodiment of the present invention.

FIG. 5 is a flow chart of illustrative steps involved with writing datainto an array of cleared memory cells of the type shown in FIG. 3 inaccordance with an embodiment of the present invention.

FIG. 6 is a diagram of an illustrative memory cell having true andcomplement clear lines in accordance with an embodiment of the presentinvention.

FIG. 7 is a flow chart of illustrative steps involved in clearing anarray of memory cells of the type shown in FIG. 6 in accordance with anembodiment of the present invention.

FIG. 8 is a flow chart of illustrative steps involved with writing datainto an array of cleared memory cells of the type shown in FIG. 6 inaccordance with an embodiment of the present invention.

FIG. 9 is a top view of an illustrative layout that may be used for amemory cell in accordance with an embodiment of the present invention.

FIG. 10 is a circuit diagram showing signal lines that may be used incontrolling a memory element array in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION

Integrated circuits often include arrays of volatile memory elements.These memory element arrays can be used to store data during dataprocessing operations. In programmable integrated circuits such asprogrammable logic devices, arrays of memory elements may be loaded withconfiguration data that is used in configuring programmable logiccircuitry. Memory cells that are used in storing configuration data forprogrammable integrated circuits are sometimes referred to asconfiguration random-access memory (CRAM) cells. Memory cells used inother types of random-access memory (RAM) arrays are sometimes referredto as RAM cells.

Memory arrays formed from CRAM cells and other RAM cells can bedisrupted by radiation strikes. Disruptions of this type are known assoft error upset events. Soft error upset events are caused whenhigh-energy atomic particles such as neutrons and alpha particles strikea portion of a memory element. Electron-hole pairs are generated when anatomic particle strikes the silicon that makes up a memory element. Theelectron-hole pairs can disrupt the charges on various nodes in thememory element and thereby cause the memory element to change states(e.g., to flip from a one to a zero or vice versa).

To reduce or eliminate soft error upset events and thereby improveintegrated circuit reliability, memory elements can be formed that havea number of redundant interconnected inverter-like circuits. Theinterconnected inverter circuits can provide signals that serve asrestoring feedback sources in the event of a radiation strike. Memoryelements with these interconnected inverter circuits are thereforeresistant to soft error upset events. Memory elements (cells) of thistype may contain any suitable number of transistors. With one suitablearrangement, each memory element may contain ten transistors.

The memory elements can be used in any suitable integrated circuits thatuse memory. These integrated circuits may be memory chips, digitalsignal processing circuits with memory arrays, microprocessors,application specific integrated circuits with memory arrays,programmable integrated circuits such as programmable logic deviceintegrated circuits in which memory elements are used for configurationmemory, or any other suitable integrated circuit. For clarity, thepresent invention may sometimes be described in the context ofprogrammable logic device integrated circuits. This is, however, merelyillustrative. Memory cells in accordance with embodiments of the presentinvention may be used in any suitable circuits. On integrated circuitssuch as memory chips or other circuits in which memory is needed tostore processing data, the memory elements may perform the functions ofstatic random-access memory (RAM) cells and are sometimes referred to asSRAM cells. In the context of programmable logic device integratedcircuits, the memory elements can be used to store configuration dataand are therefore sometimes referred to in this context as configurationrandom-access memory (CRAM) cells.

An illustrative integrated circuit 10 such as a programmable logicdevice or other programmable integrated circuit with memory is shown inFIG. 1.

Device 10 may have input/output circuitry 12 for driving signals off ofdevice 10 and for receiving signals from other devices via input/outputpins 14. Interconnection resources 16 such as global and local verticaland horizontal conductive lines and buses may be used to route signalson device 10. Interconnection resources 16 include fixed interconnects(conductive lines) and programmable interconnects (i.e., programmableconnections between respective fixed interconnects). Programmable logic18 may include combinational and sequential logic circuitry. Theprogrammable logic 18 may be configured to perform a custom logicfunction. The programmable interconnects associated with interconnectionresources may be considered to be a part of programmable logic 18.

Programmable logic device 10 contains volatile memory elements 20 thatcan be loaded with configuration data (also called programming data)using pins 14 and input/output circuitry 12. Once loaded, the memoryelements each provide a corresponding static control output signal thatcontrols the state of an associated logic component in programmablelogic 18. If desired, memory elements 20 may be used in SRAM-type memoryarrays (e.g., to store data for processing circuitry during operation ofdevice 10).

Each memory element 20 may be formed from a number of transistorsconfigured to form a bistable circuit. With one suitable approach,complementary metal-oxide-semiconductor (CMOS) integrated circuittechnology is used to form the memory elements 20, so CMOS-based memoryelement implementations are described herein as an example. If desired,other integrated circuit technologies may be used to form the memoryelements and the integrated circuit in which the memory elements areused to form memory arrays.

The memory elements may be loaded from an external erasable-programmableread-only memory and control chip or other suitable data source via pins14 and input/output circuitry 12. Loaded CRAM memory elements 20 mayprovide static control signals that are applied to the terminals (e.g.,gates) of circuit elements (e.g., metal-oxide-semiconductor transistors)in programmable logic 18 to control those elements (e.g., to turncertain transistors on or off) and thereby configure the logic inprogrammable logic 18. The circuit elements may be transistors such aspass transistors, parts of multiplexers, look-up tables, logic arrays,AND, OR, NAND, and NOR logic gates, etc.

The memory elements 20 may be arranged in an array pattern. In a typicalmodern programmable logic device, there may be millions of memoryelements 20 on each chip. During programming operations, the array ofmemory elements is provided with configuration data by a user (e.g., alogic designer). Once loaded with configuration data, the memoryelements 20 selectively control portions of the circuitry in theprogrammable logic 18 and thereby customize its functions so that itwill operate as desired.

The circuitry of device 10 may be organized using any suitablearchitecture. As an example, the logic of programmable logic device 10may be organized in a series of rows and columns of larger programmablelogic regions each of which contains multiple smaller logic regions. Thelogic resources of device 10 may be interconnected by interconnectionresources 16 such as associated vertical and horizontal conductors.These conductors may include global conductive lines that spansubstantially all of device 10, fractional lines such as half-lines orquarter lines that span part of device 10, staggered lines of aparticular length (e.g., sufficient to interconnect several logicareas), smaller local lines, or any other suitable interconnectionresource arrangement. If desired, the logic of device 10 may be arrangedin more levels or layers in which multiple large regions areinterconnected to form still larger portions of logic. Still otherdevice arrangements may use logic that is not arranged in rows andcolumns.

When memory elements 20 are arranged in an array, horizontal andvertical conductors and associated control circuitry may be used toaccess the memory elements. The control circuitry may, for example, beused to clear all or some of the memory elements. The control circuitrymay also write data into the memory elements and may read data from thememory elements. For example, in CRAM arrays, memory elements may beloaded with configuration data. Loaded configuration data may then beread out from the array to confirm proper data loading operations beforedevice 10 is used during normal operation in a system.

Any suitable memory array architecture may be used for memory elements20. One suitable arrangement is shown in FIG. 2. As shown in FIG. 2,there may be an array 28 of memory elements 20 in device 10. There areonly three rows and columns of elements 20 in the illustrative array ofFIG. 2, but in general there may be hundreds or thousands of rows andcolumns in array 28. Array 28 may be one of a number of arrays on agiven device 10, may be a subarray that is part of a larger array, ormay be any other suitable group of memory elements 20. Each memoryelement may supply a corresponding output signal OUT at a correspondingoutput path 38. In CRAM arrays, each signal OUT is a static outputcontrol signal that may be conveyed over a path 40 and used inconfiguring a corresponding transistor such as transistor 18 or othercircuit element in associated programmable logic 18.

Integrated circuit 10 may have control circuitry 24 for supplyingsignals to memory elements 20 in memory array 28. Control circuitry 24may receive power supply voltages, data, and other signals from externalsources using pins 14 and from internal sources using paths such aspaths 30. Control circuitry 24 may include circuitry such as anadjustable voltage supply (regulator 22), address decoder circuitry,address register circuitry, data register circuitry, and clear controlcircuitry. Regulator 22 may be used to produce time-varying power supplyvoltages. These power supply voltages may be of the same magnitude asthe voltages received on pins 14 or may have different magnitudes thanthe voltage received from pins 14. Control circuitry 24 (e.g., theaddressing circuitry, data register circuitry, clear control circuitry,and other control circuitry of circuitry 24) can use the power supplyvoltages supplied by pins 14 and by regulator 22 to produce desiredtime-varying and fixed signals on paths such as paths 32 and 34.

There may, in general, be any suitable number of conductive linesassociated with paths 32 and 34. For example, each row of array 28 mayhave a single associated address line in a respective one of paths 32that carries a corresponding address signal ADDR (as an example). Eachcolumn of array 28 may have a respective path 34 in which acorresponding data line (i.e., a complementary data line) is used toreceive a complementary data signal NDATA. A clear signal such as acomplementary clear signal NCLR may be routed to all of the cells inarray 28 simultaneously over a common clear line. The clear line may beoriented horizontally so that there is one branch of the clear line ineach path 32 or may be oriented vertically so that there is one branchof the clear line in each path 34. Power can also be distributed in thistype of global fashion. For example, a positive power supply voltage(sometimes referred to as Vcc) may be supplied in parallel to each cell20 using a pattern of shared horizontal or vertical conductors. A groundvoltage Vss may likewise be supplied in parallel to the cells 20 using apattern of shared horizontal or vertical lines. Address lines and datalines are typically orthogonal to each other (i.e., address lines arehorizontal while data lines are vertical or vice versa).

If desired, other patterns of lines may be used in paths 32 and 34. Forexample, both true and complement clear signals (CLR and NCLR) can berouted into array 28 using parallel sets of lines. Similarly, differentnumbers of power supply signals, data signals, and address signals maybe used.

The signals that are supplied to memory elements 20 may sometimes becollectively referred to as control signals. In particular contexts,some of these signals may be referred to as power signals, clearsignals, data signals, address signals, etc. These different signaltypes are not mutually exclusive. For example, a clear signal for array28 serves as a type of control signal that can be used to clear array28. This clear signal also serves as a type of power signal by poweringinverter-like circuitry in cells 20. Likewise, because clearingoperations serve to place logic zeros in memory cells 20, clear signalsmay serve as a type of data signal.

Any suitable values may be used for positive power supply voltage Vccand ground voltage Vss. For example, positive power supply voltage Vccmay be 1.2 volts, 1.1 volts, 1.0 volts, 0.9 volts, or any other suitablevoltage. Ground voltage Vss may be, for example, 0 volts. In a typicalarrangement, power supply voltages Vcc may be 1.0 volts, Vss may be 0volts, and the signal levels for address, data, and clear signals mayrange from 0 volts (when low) to 1.0 volts (when high).

Sometimes performance can be enhanced by temporarily raising or loweringthese voltage levels from their nominal value. For example, it may bedesirable to temporarily lower a Vcc value to weaken all or part of acell 20 during data writing operations or it may be desirable totemporarily elevate a Vcc value to strengthen all or part of a cellduring data read operations. During normal operation, it may bedesirable to elevate the power supply voltage for cells 20, as this maycorrespondingly elevate the value of the output voltage (signal OUT) onthe output lines 38 that are associated with cells 20 (i.e., for thosecells that are loaded with a logic one). The static output signals thatare provided at the output of each CRAM cell can be applied to the gateof a metal-oxide-semiconductor transistor over a corresponding controlline (e.g., control line 40 in FIG. 2). This elevated output signal may,in turn, help to fully turn on a corresponding transistor inprogrammable logic 18, improving circuit performance.

If desired, elevated logic high voltages and/or reduced ground voltagescan be used for data signals, clear signals, and address signals.Whether associated with temporarily altered power supply voltages ortemporarily elevated signal strengths for other control signals, thesetime-varying signal strength enhancement schemes are sometimes referredto as overdrive schemes. These schemes may be used in any suitablecombination. As just one example, address signal ADDR may be overdriven(e.g., to an elevated voltage of 1.2 volts) during addressing operationsfor reads and writes with array 28. Overdriving ADDR in this way mayhelp to fully turn on the address transistors in array 28, improvingperformance. In the same array 28 that uses an overdriven addresssignal, the positive power supply voltage Vcc (or a clear signal used asa power supply voltage) that is supplied to cells 20 may be temporarilyreduced in power during data write operations to enhance write margin(e.g., to 0.9 volts). This power supply voltage (or the clear signal)can then be taken to a larger value during normal operation (e.g., 1.4volts), to increase the magnitude of signal OUT (for those cells thatcontain logic ones).

In general, any suitable arrangement may be used for varying signalstrengths for array 28 (e.g., time varying power supply levels,temporarily overdrive address signal levels, etc.). These optionalsignal strength modifications may take place during clear operations inwhich array 28 is being cleared, during write operations in which datais being written into array 28, during read operations in which data isbeing read out from array 28, and during normal operation in whichsignals OUT from a loaded array are being applied to correspondingprogrammable logic 18.

When using these schemes, the magnitudes of true and complement datasignals, true and complement clear signals, and power supply signalssuch as Vcc and Vss can changed as a function of time in addition to thenormal time variations that these signals experience when transitioningfrom their logic high to logic low values. Signal magnitude variationscan be imposed on both logic high signals (e.g., by temporarily reducinga logic high value from 0.9 volts to 0.7 volts) and logic low signals(e.g., by temporarily reducing a logic low value from 0 volts to −0.2volts). Combinations of positive voltage variations and ground voltagevariations may also be made if desired.

Circuitry 24 may control clearing operations, data loading operations,and read operations for array 28.

In preparation for data loading operations, circuitry 24 may clear array28. The clear operation takes the contents of each memory cell 20 to aknown value (i.e., to a logic zero). Once cleared, each OUT signal willbe low (i.e., Vss). Circuitry 24 may produce clear signals such as acomplementary clear signal NCLR and/or a true clear signal CLR that areused in clearing operations. When clearing a CRAM array, all clear linesof a given type (e.g., all complementary clear lines in an array that iscleared using only complementary clear lines) may be commonlycontrolled. In this way, the clear signals may be asserted anddeasserted simultaneously for all cells in the array. This type ofglobal clear operation can help to reduce clear times. If desired, clearsignals can be asserted individually for different groups of cells.Clearing operations for CRAM arrays are typically performed upon systempower-up or during reconfiguration.

After array 28 has been cleared, circuitry 24 may load data into array28. Circuitry 24 may receive data such as configuration data fromexternal sources via input path 30. In a typical programmable logicdevice system, configuration data is loaded into a programmable logicdevice from a memory and data loading circuit. This type of circuit,which is sometimes referred to as a configuration device, loadsconfiguration data into registers within circuitry 24. Address decodercircuits in circuitry 24 may receive external control signals oraddressing control signals can be generated internally in circuitry 24.Address signals can be controlled independently in each row (or incolumn-based addressing schemes, in each column).

Configuration data may be loaded into registers in circuitry 24 inseries. These registers may then apply the configuration data inparallel to array 28 via data lines (e.g., true data lines,complementary data lines, or both true and complementary data lines).Address decoder circuitry in circuitry 24 can receive addressinginformation via input 30. The address decoder circuitry can thensystematically assert desired address lines 32. As the address line ineach row is asserted (i.e., as the signal ADD in a given row is takenhigh), the data on the data lines 34 is loaded into the memory elements20 in that column. By addressing each row in this way, the entire array28 may be loaded with configuration data. After the array has beenloaded, the output 38 of each memory element 20 produces a correspondingstatic control signal for controlling the gate of a pass transistor orother logic component in the programmable logic 18 of the programmablelogic device 10 (FIG. 1).

Control circuitry 24 may perform data read operations on array 28 toconfirm that data has been loaded properly. Data reading operations maybe performed by systematically asserting desired address lines andmonitoring the resulting data on data lines (e.g., using sense amplifiercircuitry in circuitry 24).

A memory element 20 of the type that may be used in array 28 of FIG. 2is shown in FIG. 3. As shown in FIG. 3, memory element 20 may have a tentransistor (10T) configuration and may include p-channelmetal-oxide-semiconductor (PMOS) transistors P1, P2, P3, and P4 andre-channel metal-oxide-semiconductor (NMOS) transistors N1, N2A, N2B,N3, N4A, and N4B. The gates of these transistors are labeled “G.” Thesources S and the drains D are also labeled. The source and drainterminals of the transistors in element 20 are sometimes collectivelyreferred to as source-drain terminals.

Data can be conveyed over data lines such as complement data lines 34(i.e., in the form of complement data signals NDATA). An associatedaddress line such as address line 32A may be used to convey addresssignal ADDR to the gates of address transistors N2A and N4A. Clear linessuch as complement clear lines 32B may be used to provide memory element20 with clear signals (i.e., complement clear signal NCLR). A positivepower supply signal (e.g., Vcc) may be applied to positive power supplyterminals 42. A ground power supply signal (e.g., Vss) may be applied toground terminals 44.

There are four labeled nodes in memory element 20 of FIG. 3: 1, 2, 3,and 4. Any of these nodes may be used as output 38 of FIG. 2. Forexample, node 1 may be connected to output 38 of FIG. 2, as shown in theexample of FIG. 3. Multiple outputs may also be tapped from two or moreof the nodes in a single cell if desired.

Memory element 20 exhibits bistable operation. When memory element 20has been loaded with a “1,” the values of 1, 2, 3, and 4 will be “1,”,“0,” “1,” and “0,” respectively. When memory element has been loadedwith a “0,” the values of 1, 2, 3, and 4 will be “0,”, “1,” “0,” and“1,” respectively.

Unlike conventional memory element designs that are based on a pair ofcross-coupled inverters, memory element 20 of FIG. 3 has fourinverter-like pairs of transistors (sometimes referred to as“inverters”) that are connected in a ring to form a bistable memoryelement. A first pair of transistors (P1 and N1) forms inverter-likecircuit INV1 and is associated with output node 1. A second pair oftransistors (P2 and N2B) forms inverter-like circuit INV2 and isassociated with output node 2. Likewise, a third pair of transistors (P3and N3) forms inverter-like circuit INV3 having node 3, whereas a fourthpair of transistors (P4 and N4B) forms inverter-like circuit INV4 havingnode 4.

In conventional cross-coupled inverter designs, there is a relativelystrong likelihood that a radiation strike on an inverter's input nodewill cause the inverter's output to change states, thereby flipping thestate of the cell to an erroneous value.

The FIG. 3 layout uses four interlocked inverter circuits to produce astable cell that exhibits good immunity to soft error upset events. Inthe arrangement of FIG. 3, the gate of the p-channel transistor in eachtransistor pair receives its input from a different output node than thegate of the n-channel transistor in the same transistor pair. Forexample, node 1, which is located between the drain D of transistor P1and the drain D of transistor N1 in inverter INV1 produces a controlsignal that is passed to the gate G of transistor N4B in inverter INV4.The gate G of the other transistor in inverter INV4 (i.e., the gate G oftransistor P4) receives a control signal from a different inverter. Inparticular, gate G of transistor P4 receives a gate control signal fromnode 3 in inverter INV3. Similarly, the gate control signals for thetransistors in inverter INV1 are received from nodes 2 and 4, gateinputs for the transistors in inverter INV2 are received from nodes 1and 3, and the gate inputs for the transistors in inverter INV3 arereceived from nodes 2 and 4. Because the control signals for thetransistor gates of each inverter are received from the outputs of twodifferent inverters, the control signals are distributed betweeninverters and are less dependent on the stability of any given inverter.With the control signals distributed in this way, memory cell 20 isbetter able to recover from a radiation strike on a particular node(i.e., one of the four inverters) without flipping its state thanconventional memory elements based on a pair of cross-coupled inverters.

The ten-transistor configuration of memory element 20 of FIG. 3 providesgood stability and resistance to undesired upsets from radiationstrikes. Nodes 2 and 4 can be accessed for data write and readoperations using address transistors N2A and N4A. The addresstransistors in memory element 20 may, in general, be n-channelmetal-oxide-semiconductor transistors, p-channelmetal-oxide-semiconductor transistors, or may include both n-channel andp-channel transistors. In the illustrative arrangement of FIG. 3,address transistors N2A and N4A are n-channel transistors. When drivingsignals through an n-channel transistor (i.e., from source to drain),low voltages (i.e., logic zeros at Vss) are more effectively passed thanhigh voltages (i.e., logic ones at Vcc). This is because re-channeladdress transistors are more fully turned on when NDATA is low and ADDRis high than when NDATA is high and ADDR is high. As a result, DATA “1”values can effectively be loaded through address transistors N2A and N4Awhile circuits INV1, INV2, INV3, and INV4 are all powered normally. ButDATA “0” values can generally only be placed in memory element 20, byperforming a clear operation in which the NCLR lines 32B are used inselectively depowering part of the memory element.

Consider a scenario in which it is desired to use address transistorsN2A and N4A to load a logic one into memory element 20. To load a logicone value, complement data signals NDATA are held low at Vss on lines34. Address signal ADDR on line 32A is taken high. When asserting ADDRin this way, the gates G of address transistors N2A and N4A are held atVcc while the drains D of address transistors N2A and N4A are held atVss by lines 34. In this situation, address transistors N2A and N4A willturn on and will pull their source terminals S to Vss. This pulls nodes2 and 4 low. Because of the feedback provided by the interconnectedgates of memory element 20, pulling nodes 2 and 4 low will drive nodes 1and 3 high to Vcc. Once nodes 2 and 4 are low and nodes 1 and 3 arehigh, memory element 20 will be in a stable state storing a logic oneand memory address line 32A may be deasserted. When storing a logic onevalue for DATA, output signal OUT on line 38 will be high.

Driving NDATA lines 34 to Vcc is not typically sufficient to clearmemory element 20, because logic one values are not passed stronglyenough through n-channel address transistors N2A and N4A. If desired,one or more additional address transistors could be added to memoryelement 20 to directly address nodes 1 and 3. Adding such addresstransistors would, however, increase the area consumed by each memoryelement 20. Therefore, memory element 20 is preferably provided withonly two address transistors (transistors N2A and N4A). The challengesfaced by address transistors N2A and N4A in placing a logic zero inmemory element 20 by taking NDATA high can be overcome by tying thesources S of transistors P1 and P3 to NCLR lines 32B as shown in theillustrative layout of FIG. 3.

With this type of arrangement, clear control signal NCLR can be used inclearing all memory elements 20 in array 28 simultaneously. Because thesources S of transistors P1 and P3 are tied to NCLR, current surgesthrough transistors P1 and P3 (and therefore through inverter circuitsINV1 and INV3) can be avoided by holding NCLR low during clearoperations. Reduced currents of this type, which are sometimes referredto as cross-bar currents, help to reduce the need for large powersupplies and large signal distribution lines on integrated circuit 10,thereby helping to reduce cost and complexity for circuit 10.

Even though memory element 20 does not have address transistorsconnected to nodes 1 and 3, nodes 1 and 3 can be taken low to clearmemory element 20 during clear operations by holding NCLR at Vss whileasserting address signal ADDR (i.e., to a normal Vcc level or anelevated power supply voltage). During clear operations, the lowvoltages on complement clear control signal NCLR depower inverter-likecircuits INV1 and INV3, which makes the state change easier (flippingDATA from high to low).

Illustrative operations involved in clearing an array 28 of memoryelements 20 of the type shown in FIG. 3 are shown in FIG. 4.

At step 46, control circuitry 24 (FIG. 2) can take NCLR to Vss. Thisoperation can be performed globally, using a set of distribution linesthat are tied to a common output node for circuitry 24 (i.e., a commonNCLR line). Taking NCLR low pulls nodes 1 and 3 towards Vss (i.e., thisoperation pulls nodes 1 and 3 low).

At step 48, control circuitry 24 can take signal NDATA high. This takesthe drains D of address transistors N2A and N4A high. NDATA can be takenhigh in all columns for which clearing is desired (e.g., for all columnsin array 28).

At step 50, control circuitry 24 can assert signal ADDR for all rows forwhich clearing is desired (e.g., for one or more rows in array 28).Address signal ADDR can be asserted by taking ADDR to a normal positivepower supply level such as the level used for data signals in normalprogrammable logic or other circuitry on integrated circuit (e.g., anominal Vcc value) or can be overdriven to an elevated value. Forexample, if the nominal power supply voltage Vcc on device 10 is 0.9volts, ADDR can be set to 0.9 volts or a higher value such as 1.1 volts(as an example).

When ADDR is asserted, the gates G of address transistors N2A and N4Aare taken high. This turns on address transistors N2A and N4A. Whentransistors N2A and N4A are turned on, drains D of transistors N2A andN4A are shorted to respective sources S of transistors N2A and N4A. As aresult, nodes 2 and 4 are pulled high towards the high NDATA voltagelevel. Because circuits INV1 and INV3 are depowered, transistors N1 andN3 are turned on as nodes 2 and 4 go high, holding nodes 1 and 3 firmlyat Vss. In this situation, the gates of transistors P2 and P4, whichreceive control signals from nodes 1 and 3, respectively, are low andtransistors P2 and P4 are fully on, pulling complementary data nodes 2and 4 high. The address signal ADDR may then be deasserted (step 52) andinverters INV1 and INV3 can be repowered by returning NCLR to Vcc (step54). The NDATA signals can be maintained at Vcc. As inverter-likecircuits INV1 and INV3 are powered, the high value of node 4 holdstransistor P1 off, so that node 1 remains low. Similarly, the high valueof node 2 holds transistor P3 off, so that node 3 remains low. Followingstep 54, each memory element 20 is in a stable state with DATA equal tozero (i.e., nodes 2 and 4 are high and nodes 1 and 3 are low). Allmemory elements 20 in array 28 have therefore been cleared.

After array 28 has been cleared so that the memory elements 20 in array28 each contain a logic zero, array 28 may be loaded with configurationdata or other suitable data values. In particular, data write operationsmay be performed in which logic one values are written into the array atdesired locations. During data write operations, appropriate signalsNDATA are held low. These low values pass through the addresstransistors of addressed cells 20 and serve to pull complementary datanodes 2 and 4 low. Once loading is complete, each cell into which a lowNDATA value was loaded will be loaded with a logic one.

Illustrative steps involved in writing logic ones into an array 28 ofmemory elements 20 of the type shown in FIG. 3 are shown in FIG. 5.

Initially, memory elements 20 in array 28 may be provided with high NCLRvalues (step 56) as during normal operation of array 28. A globaldistribution path (i.e., a path in which all clear control signaldistribution lines are tied to a common node) can be used indistributing the NCLR values to all of the memory elements in array 28in parallel. With NCLR high (e.g., at Vcc), inverter-like circuits INV1and INV3 will be powered in each memory element. Inverter-like circuitsINV2 and INV4 are powered using the positive power supply voltage (e.g.,Vcc) that is applied to power supply terminal 42.

At step 58, control circuit 24 may take complement data signals NDATAlow for certain columns in array 28 to prepare for the loading of dataone values into the cells of an addressed row of array 28. The state ofNDATA in each column may be determined by the output of a data registerin control circuitry 24. Using this approach, the values of NDATA may behigh for some columns of array 28 and may be low for other columns ofarray 28. In general, any suitable number of NDATA signals may be heldlow. Arrangements in which control circuit 24 holds NDATA low for morethan one column at the same time allow data one values to be writteninto multiple memory elements 20 in parallel.

At step 60, control circuit 24 can take the address signal ADDR high ina given row of array 28. The address transistors N2A and N4A in thosememory elements 20 for which NDATA is low are therefore turned on. Inthese memory elements 20, the sources S of the address transistors areshorted to their drains D, pulling nodes 2 and 4 low. When nodes 2 and 4are pulled low in this way, transistor P1 in circuit INV1 will be turnedon and transistor N1 will be turned off, taking node 1 high, whereastransistor P3 in circuit INV3 will be similarly turned on and transistorN3 will be turned off, taking node 3 high. The address signal ADDR inthe given row can then be deasserted (e.g., taken low) at step 62.

As a result of these operations, all memory elements 20 corresponding tolow NDATA signals in the row in which ADDR was asserted will be loadedwith logic ones. The memory elements in the same row that correspond tohigh NDATA signals will remain cleared (loaded with logic zeros).

If more rows in array 28 remain to be loaded with logic ones, processingcan loop back to step 58, as indicated by line 64. If all desired memoryelements 20 in array 28 have been loaded, data write operations arecomplete and integrated circuit 10 can be used in a system (step 66). Insituations in which integrated circuit 10 is a programmable integratedcircuit and in which memory array 28 is loaded with configuration data,each loaded memory element 20 may provide a corresponding static outputcontrol signal that configures a transistor or other component inprogrammable logic 18. During the operations of step 66, theprogrammable logic that has been customized in this way may be used toprocess system signals.

Cross-bar current surges during clear operations can be further reducedby momentarily depowering inverter-like circuits INV2 and INV4 inaddition to depowering inverter-like circuits INV1 and INV3. Anillustrative memory element 20 that includes true clear control lines32C for selectively depowering inverters INV2 and INV4 during clearoperations to help further reduce current surges in this way is shown inFIG. 6.

As shown in FIG. 6, memory element 20 may have interconnectedinverter-like circuits INV1, INV2, INV3, and INV4 of the type shown inFIG. 3. Circuits INV1 and INV3 of FIG. 6 may be powered by thecomplement clear control signal NCLR on path 32B and the ground signalVss on ground terminals 44, as with circuits INV1 and INV3 of FIG. 3.Circuits INV2 and INV4 of FIG. 6 may be powered using a positive powersupply (e.g., Vcc) on terminals 42 and true clear control signal CLR onpath 32C and terminals 68. In this type of configuration, the complementclear control signal NCLR serves as a controllable positive power supplyvoltage that can be used to selectively power and depower inverter-likecircuits INV1 and INV3. The true clear control signal CLR serves as acontrollable ground power supply signal that can be used to selectivelypower and depower inverter-like circuits INV2 and INV4.

Illustrative operations involved in clearing an array 28 of memoryelements 20 of the type shown in FIG. 6 are shown in FIG. 7.

At step 70, control circuitry 24 of FIG. 2 can take NCLR to Vss and cantake CLR to Vcc. This temporarily depowers circuits INV1, INV2, INV3,and INV4. The operations of step 70 may be performed globally. Inparticular, a global distribution network may be used to distributesignal NCLR to all memory elements 20 in array 28 in parallel.Similarly, a global distribution network may be used to distributesignal CLR to all memory elements 20 in array 28 in parallel. TakingNCLR low pulls nodes 1 and 3 towards Vss (i.e., this operation pullsnodes 1 and 3 low), whereas taking CLR high pulls nodes 2 and 4 towardsVcc (i.e., this operation pulls nodes 2 and 4 high).

At step 72, control circuitry 24 can take signal NDATA high in allcolumns for which clearing is desired (e.g., for all columns in array28). This takes the drains D of address transistors N2A and N4A high.

At step 74, control circuitry 24 can assert signal ADDR for all rows forwhich clearing is desired (e.g., for all rows in array 28). As with theclearing operations of FIG. 4, all rows in array 28 can besimultaneously cleared or a subset of the rows in array 28 can besimultaneously cleared (e.g., one row at a time or in groups).

During step 74, signal ADDR can be asserted by taking ADDR to a normalpositive power supply level such a the level used for normalprogrammable logic or other circuitry on integrated circuit 10 (e.g., anominal Vcc value) or can be overdriven to an elevated value. Forexample, if the nominal power supply voltage Vcc on device 10 is 0.9volts, ADDR can be set to 0.9 volts or a higher value such as 1.1 volts(as an example).

When ADDR is asserted, the gates G of address transistors N2A and N4Aare taken high, turning on address transistors N2A and N4A. This shortsthe drains D of transistors N2A and N4A to the respective sources S oftransistors N2A and N4A. Nodes 2 and 4 are therefore pulled high to thehigh NDATA voltage level. Transistors N1 and N3 are turned on as nodes 2and 4 are pulled high, shorting nodes 1 and 3 to Vss on ground terminals44. The gates of transistors P2 and P4 receive respective controlsignals from nodes 1 and 3 and are therefore taken low. The low gatevoltages for transistors P2 and P4 turn transistors P2 and P4 on andshort nodes 2 and 4 to positive power supply terminals 42, pulling nodes2 and 4 high. In this situation, nodes 1 and 3 are low and nodes 2 and 4are high (i.e., the memory cells have all been cleared).

The clear operations involve negligible amounts of cross-bar current,because NCLR, which served as the positive power supply voltage forinverters INV1 and INV3 was low during the clear operation, and becauseCLR, which served as the ground power supply voltage for inverters INV2and INV4 was high during the clear operation. Eliminating current surgesduring clear operations helps to reduce the current handlingrequirements of circuitry such as circuitry 24 of FIG. 2 and therebyhelps minimize the cost and complexity of circuitry 24. There isgenerally less current during the clear operations of FIG. 7 than duringthe clear operations of FIG. 4, because in the FIG. 7 arrangement bothsets of inverter-like circuits (i.e., both INV1/INV3 and INV2/INV4) areprevented from drawing significant cross-bar current during clearing,whereas in the FIG. 4 arrangement only the INV1/INV3 circuits wereblocked from drawing current.

After memory elements 20 have been cleared, the clear operations of FIG.7 can be completed by deasserting address signal ADDR (step 76) and byreturning memory elements 20 to their normal powered state. Inparticular, during the operations of step 78, inverter-like circuitsINV1 and INV3 can be repowered by returning NCLR to Vcc andinverter-like circuits INV2 and INV4 can be repowered by returning CLRto Vss. Following step 78, each memory element 20 is in a stable statewith DATA equal to zero (i.e., nodes 2 and 4 are high and nodes 1 and 3are low). All memory elements 20 in array 28 have therefore beencleared.

After array 28 has been cleared, array 28 may be loaded withconfiguration data or other suitable data. In particular, data writeoperations may be performed in which logic ones are written into array28 at desired locations. When writing a logic one into a memory element,the signals NDATA for that memory element are held low. These low valuespass through the address transistors of the memory element 20 and serveto pull complementary data nodes 2 and 4 low for that memory element.The low values of nodes 2 and 4, in turn, pull nodes 1 and 3 high,completing the loading of the data one value.

Illustrative steps involved in writing logic ones into an array 28 ofmemory elements 20 of the type shown in FIG. 6 are shown in FIG. 8.

Memory elements 20 in array 28 are initially provided with high NCLRvalues and low CLR values by control circuit 24 as during normaloperation of array 28 (step 80). Global distribution paths may be usedin distributing signals CLR and NCLR. For example, a global true clearcontrol signal distribution path may be used to globally distribute trueclear control signal CLR to all memory elements 20 in parallel and aglobal complement clear control signal distribution path may be used toglobally distribute complement control signal CLR to all memory elements20 in parallel. With NCLR high and CLR low, inverter-like circuits INV1,INV2, INV3, and INV4 will be powered normally in each memory element 20.

At step 82, control circuit 24 may take complement data signals NDATAlow for certain columns in array 28 to prepare for the loading of dataone values into the cells of an addressed row of array 28. The state ofNDATA in each column may be determined by the output of a data registerin control circuitry 24. Using this approach, the values of NDATA may behigh for some columns of array 28 and may be low for other columns ofarray 28. In general, any suitable number of NDATA signals may be heldlow. Arrangements in which control circuit 24 holds NDATA low for morethan one column at the same time allow data one values to be writteninto multiple memory elements 20 in parallel.

At step 84, control circuit 24 can take the address signal ADDR high ina given row of array 28. The address transistors N2A and N4A in thosememory elements 20 for which address signals ADDR is asserted and forwhich NDATA is low will be turned on. In these memory elements 20, thesources S of the address transistors are shorted to their drains D,pulling nodes 2 and 4 low. When nodes 2 and 4 are pulled low, transistorP1 in circuit INV1 will be turned on and transistor N1 will be turnedoff, taking node 1 high, while transistor P3 in circuit INV3 will beturned on and transistor N3 will be turned off, taking node 3 high. Theaddress signal ADDR in the given row can then be deasserted (e.g., takenlow) at step 86.

As a result of these operations, all memory elements 20 corresponding tolow NDATA signals in the row in which ADDR was asserted will be loadedwith logic ones. The memory elements in the same row that correspond tohigh NDATA signals will remain cleared (loaded with logic zeros).

If more rows in array 28 remain to be loaded with logic ones, processingcan loop back to step 82, as indicated by line 88. If all desired memoryelements 20 in array 28 have been loaded, data write operations arecomplete and integrated circuit 10 can be used in a system (step 90). Insituations in which integrated circuit 10 is a programmable integratedcircuit and in which memory array 28 is loaded with configuration data,each loaded memory element 20 may provide a corresponding static outputcontrol signal that configures a transistor or other component inprogrammable logic 18. The programmable logic that has been customizedin this way may be used during the operations of step 90 to processsystem signals.

Immunity to soft error upset events may be enhanced by spatiallyseparating the circuit components that may be subject to disturbance bya radiation strike. In particular, soft error upset immunity may beenhanced by forming transistor P1 and its redundant partner transistorP3 in different semiconductor regions (i.e., different n-wells). Otherredundant transistor pairs (e.g., transistors P2 and P4, N1 and N3, N2and N4) may likewise be isolated from each other by forming thetransistors in these redundant pairs in respective wells (semiconductorregions). With this type of distributed arrangement, radiation strikesthat disrupt the operation of one of these transistors will affect thattransistor, but not its redundant partner.

For example, when a radiation strike arises in the n-well that containstransistor P1, the operation of transistor P1 can be disrupted by theelectron-hole pairs and subsequent minority carrier diffusion thatresults from the radiation strike in the n-well. If transistor P3 wereformed in the same n-well, these diffused carriers could adverselyaffect transistor P3 at the same time. The radiation strike wouldtherefore have the potential to disrupt not only node 1 at the output oftransistor P1, but node 3 at the output of transistor P3, therebypotentially causing memory element 20 to flip states. If, on the otherhand, transistors P1 and P3 are isolated by forming transistors P1 andP3 in different n-wells, a radiation strike on transistor P1 couldaffect the signal on node 1, but would not directly affect the signal onnode 3. The undisturbed value of the node 3 signal would therefore serveas a stabilizing signal that helps to restore memory element 20 to itsoriginal state even in the presence of the strike on transistor P1.

In memory cell 20, transistors P1 and P3 form a redundant pair and canbe isolated using separate semiconductor regions. Transistors P2 and P4likewise form a redundant pair and can be isolated using separatesemiconductor regions. Transistors N1 and N3 are redundant in the sameway and can be isolated by forming each of these transistors in aseparate semiconductor regions. Isolated semiconductor regions may alsobe used in forming transistors N2B and N4B. The semiconductor region inwhich a metal-oxide-semiconductor transistor is formed is typically ann-well (for PMOS transistors) or a p-well (for NMOS transistors). Insome situations, the semiconductor region in which a transistor isformed is part of a larger semiconductor substrate region (e.g., ap-type substrate), rather than a doped well. More typically(particularly for PMOS devices), transistors are formed in smaller wellstructures. Doped semiconductor wells can be formed using ionimplantation, diffusion, or other suitable semiconductor fabricationtechniques.

An illustrative layout that may be used in forming the transistors of amemory element such as the memory elements of FIGS. 3 and 6 is shown inFIG. 9. As shown in the integrated circuit top view of FIG. 9, memoryelements 20 may be formed in one or more vertical stripes on integratedcircuit 10. Each vertical stripe may include stripe-shaped semiconductorregions 92, 94, 96, 98, and 100. Regions 92, 96, and 100 may be p-typedoped semiconductor regions. Regions 94 and 98 may be n-type dopedsemiconductor regions. The depth of these doped semiconductor regions(into the page) may be a fraction of a micron to a micron or more, asdetermined by the semiconductor fabrication process that is being usedto fabricate integrated circuit 10.

When the substrate on which integrated circuit 10 is being fabricated isa p-type silicon substrate, semiconductor regions 92, 96, and 100 may,if desired, be formed from regions of p-type silicon substrate. N-typesemiconductor regions 94 and 98 are typically formed byion-implantation, which forms n-type wells within the p-type substrate.

The sources and drains of the transistors of memory elements 20 may beformed from highly doped ion implantation regions (as an example) thatare fabricated within the doped semiconductor regions 92, 94, 96, 98,and 100. Some of the gates, sources, and drains of the transistors of agiven memory element 20 are labeled in FIG. 9. For example, transistorN1 has a gate GN1 and has a source SN1 and drain DN1 formed from heavilydoped n-type regions within p-type semiconductor region 92.

As the illustrative layout of FIG. 9 demonstrates, isolation betweenredundant transistors can be enhanced by forming each of the transistorsin a redundant pair in a separate doped semiconductor region. Forexample, source SN1 and drain DN1 of transistor N1 are formed withinp-type region 92, whereas the source and drain of its redundant partnerN3 are formed within a separate p-type semiconductor region (region 96).These regions are spatially distant from each other and are separated byan interposed region of opposite doping type (i.e., n-well 94), sominority carriers that are formed by a radiation strike in one region(e.g., region 92) will not diffuse into the other region (e.g., region96). The other redundant transistor pairs in memory element 20 maysimilarly be formed in separate semiconductor regions to enhance thestability of memory element 20 during soft error upset events.

FIG. 10 shows an illustrative set of signal distribution paths that maybe used in routing clear signals, data signals, and address signalsbetween control circuitry 24 and memory elements 20 in array 28. Asshown in FIG. 10, clear signals such as true clear signal CLR andcomplement clear signal NCLR may be globally distributed using globaldistribution paths such as paths 32C and 32B. Address signals may bedistributed to multiple memory elements in a row. In the FIG. 10example, address signal ADDR1 is distributed to each of the memoryelements 20 in the first row of array 28, address signal ADDR2 isdistributed to each of the memory elements 20 in the second row of array28, and address signal ADDR3 is distributed to each of the memoryelements 20 in the third row of array 28. Data signals NDATA1, NDATA2,and NDATA3 are conveyed between circuitry 24 and respective columns ofmemory elements 20 using data lines 34 (i.e., complement data lines).

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention.

What is claimed is:
 1. A method for operating a memory element thatincludes a first inverting circuit with transistors coupled in seriesbetween first and second power supply lines and that includes a secondinverting circuit with transistors coupled in series between third andfourth power supply lines, the method comprising: deactivating the firstand second inverting circuits by asserting control signals on the firstand third power supply lines; and while the control signals areasserted, loading a first value into the memory element.
 2. The methoddefined in claim 1, wherein asserting the control signal on the firstpower supply line comprises biasing the first and second power supplylines to identical voltage levels.
 3. The method defined in claim 1,further comprising: activating the first and second inverting circuitsby deasserting the control signals on the first and third power supplylines.
 4. The method defined in claim 1, further comprising: while thecontrol signals are deasserted, loading a second value that is differentthan the first value into the memory element.
 5. The method defined inclaim 1, wherein loading the first value into the memory elementcomprises loading the first value into the second inverting circuit. 6.The method defined in claim 1, wherein the memory element furthercomprises an access circuit that is connected to the second invertingcircuit but not to the first inverting circuit, and wherein loading thefirst value into the memory element comprises activating the accesscircuit to load the first value into the memory element.
 7. The methoddefined in claim 1, wherein asserting the control signals on the firstand third power supply lines comprises applying a first power supplyvoltage onto the first power supply line and applying a second powersupply voltage that is different than the first power supply voltageonto the second power supply line.
 8. A method for operating a memoryelement that includes at least one inverting circuit having first andsecond transistors coupled in series between first and second powersupply lines, the method comprising: powering the inverting circuit bysupplying the first and second power supply lines with power supplyvoltages; and receiving a signal that is not actively driven with thefirst transistor in the inverting circuit.
 9. The method defined inclaim 8, further comprising: receiving another signal that is notactively driven with the second transistor in the inverting circuit. 10.The method defined in claim 8, wherein powering the inverting circuitcomprises supplying power supply voltages with different voltage levelsto the first and second power supply lines.
 11. The method defined inclaim 8, further comprising: deactivating the inverting circuit bysupplying power supply voltages with identical voltage levels to thefirst and second power supply lines.
 12. The method defined in claim 8,wherein the memory element further includes an additional invertingcircuit, the method further comprising: with the additional invertingcircuit, providing the signal to the first transistor.
 13. The methoddefined in claim 12, wherein the additional inverting circuit includesthird and fourth transistors coupled in series between third and fourthpower supply lines, the method further comprising: during a first timeperiod, deactivating the additional inverting circuit by biasing thethird and fourth power supply lines to identical voltage levels toprovide the signal that is not actively driven to the first transistor.14. The method defined in claim 13, further comprising: during a secondtime period that is different than the first time period, biasing thethird and fourth power supply lines to different voltage levels toprovide a signal that is actively driven to the first transistor. 15.Memory element circuitry, comprising: first and second power supplylines on which fixed power supply voltages are provided; a third powersupply line on which a first adjustable control signal is provided; afourth power supply line on which a second adjustable control signal isprovided; a first inverting circuit having transistors coupled in seriesbetween the first and third power supply lines, wherein the firstinverting circuit has an output on which a data bit is stored; and asecond inverting circuit that is coupled to the first inverting circuit,wherein the second inverting circuit has transistors coupled in seriesbetween the second and the fourth power supply lines, and wherein thesecond inverting circuit has an output on which a data bit having thesame value of the data bit at the output of the first inverting circuitis stored.
 16. The memory element circuitry defined in claim 15, whereinthe first and second power supply lines carry different power supplyvoltages.
 17. The memory element circuitry defined in claim 15, furthercomprising: an address circuit coupled to the second inverting circuit,wherein no address circuit is connected to the first inverting circuit.18. The memory element circuitry defined in claim 15, furthercomprising: a third inverting circuit that is coupled to the secondinverting circuit, wherein the third inverting circuit have transistorscoupled in series between the first and third power supply lines. 19.The memory element circuitry defined in claim 15, further comprising:control circuitry that is configured to assert the first adjustablecontrol signal to deactivate the first inverting circuit while loading afirst value into the memory element.
 20. The memory element circuitrydefined in claim 19, wherein the control circuitry is further configuredto deassert the first adjustable control signal to activate the firstinverting circuit while loading a second value that is different thanthe first value into the memory element.
 21. The memory elementcircuitry defined in claim 19, wherein the control circuitry is furtherconfigured to assert the second adjustable control signal on the fourthpower supply line to deactivate the second inverting circuit whileloading the first value into the memory element.